Complex behaviors require that an animal can sense the external environment, integrate information and respond with the appropriate motor output. Therefore, understanding how complex behaviors are specified requires an integrated understanding of the development of the neural circuits that underlie these behaviors, as well as how physiological changes direct the adult behavioral output. Drosophila melanogaster has been a premiere model system for the study of the genetic basis of neural development and more recently for the genetic basis of complex behavior. Sophisticated molecular-genetic tools, coupled with cell imaging approaches are available for the study of Drosophila behavior and neural development. Furthermore, there is knowledge of the genetic determinants of some complex behaviors, including those that specify the neural substrates necessary for reproductive behaviors. Drosophila male courtship behaviors are genetically specified behaviors that are downstream of the sex determination hierarchy, a pre-mRNA splicing cascade that culminates in the production of sex-specific transcription factors encoded by fruitless and doublesex. Molecular-genetic studies have shown that fruitless is both necessary and sufficient for nearly all aspects of male courtship behaviors. The neurons in which the male-specific products of fruitless are expressed have been identified and shown to underlie the capacity for male courtship behaviors. This confluence of knowledge of male-specific transcription factors that specify behaviors, the identification of the neural circuit important for the behavior and the sophisticated molecular-genetic tools available for Drosophila genomic and cell imaging studies provides an unprecedented opportunity to gain insight into complex behaviors. The aims of this grant proposal are to study how sexual dimorphism in the nervous system is specified during development and how experience-dependent changes are encoded into the nervous system at adult stages. These studies will provide a foundation for understanding the genetic basis of behaviors in other animals, including humans.